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GRAIN BOUNDARY ENGINEERING PROGRESS Researchers from the Max Planck Institute for Iron Research (MPIE), Germany, along with colleagues from Northwestern University and the Leibniz Institute for Solid State and Materials [...]

Grain boundaries in graphene are formed by the joining of islands during the initial growth stage, and these boundaries govern transport properties and related device performance (1,2). Although information on [...]

Polycrystalline materials are composites of crystalline particles or 'grains' separated by thin 'amorphous' grain boundaries (GBs). Although GBs have been exhaustively investigated at low temperatures, at which these regions are relatively ordered, much less is known about them at higher temperatures, where they exhibit significant mobility and structural disorder and characterization methods are limited. The time and spatial scales accessible to molecular dynamics (MD) simulation are appropriate for investigating the dynamical and structural properties of GBs at elevated temperatures, and we exploit MD to explore basic aspects of GB dynamics as a function of temperature. It has long been hypothesized that GBs have features in common with glass-forming liquids based on the processing characteristics of polycrystalline materials. We find remarkable support for this suggestion, as evidenced by string-like collective atomic motion and transient caging of atomic motion, and a non-Arrhenius GB mobility describing the average rate of large-scale GB displacement. glass formation | grain-boundary mobility | molecular dynamics | polycrystalline materials | string-like collective motion

The several effects of disorder on the performance of two-band magnesium diboride (Mg[B.sub.2]) microwave resonators are studied. The results reveal that the grain boundaries are mainly responsible for the nonlinear behavior of the resonators.

The effect of the high aspect ratio grains on the improvement of the critical current densities of coated conductors is demonstrated. The results prove that an increase in the grain aspect ratio also increases the transport currents along the tape direction.

A new multi-seeding process for the fabrication of single Y-Ba-Cu-O (YBCO) grains containing strong artificial grain boundaries with high critical current densities is described. The seed alignment is found to be the major factor behind the strong coupling of these grain boundaries.

The presence of various flux pinning centers in the [MgB.sub.2] thin films, which are prepared by an electron beam evaporation technique, is described. The grain boundaries, as well as the MgO nanoparticles in the films are found to act as the pinning centers.

The various effects of the two-stage heat treatment on superconducting properties for in-situ [MgB.sub.2X] tapes that are prepares using Mg flakes, are being explained. The two-stage heat treatment is found to enhance the core density, as well as the inter-granular connectivity of these tapes.

The strong enhancement of critical current density in [MgB.sub.2] bulk and Ni-sheathed tapes with very small [Dy.sub.2][O.sub.3] additions is being studied. The analysis shows that the N-sheathed tapes exhibit superior critical current, as compared to the pure [MgB.sub.2] powders.

The observations of macroscopic quantum tunneling and energy level quantization in Y[Ba.sub.2][Cu.sub.3][O.sub.7]-[delta] (YBCO)-based grain boundary (GB) biepitaxial Josephson junctions which could help to understand the mechanism involved in transport across grain boundaries is reported.

The effect of dislocations, grain boundaries and bulk recombination on minority carrier lifetime in multicrystalline silicon were studied by developing a new model. Minority carrier lifetime was measured using the carrier density imaging (CDI) technique, grain boundaries were then mapped and characterized by electron backscatter diffraction (EBSD) and sub grain boundaries were found to be recombination active and dominating the number of active grain boundaries.

Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of [sup.44]Ca from a layer of [sup.44]Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900[degrees]C. A region about 500 [micro]m wide adjacent to the powder layer is heterogeneously enriched in [sup.44]Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in [sup.44]Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature, Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width ([D.sub.GB][delta]) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value of [D.sub.GB][delta] in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than regions of anomalous mineralogy, water- or melt-content as is generally suggested.

The first atomically detailed simulations are presented that examine interfacial mass transfer resistance due to internal grain boundaries in zeolites that are relevant for membrane applications. The dual control volume grand canonical molecular dynamics method is used to stimulate the permeance of CH4 and CF4 through thin twinned silicalite crystals.

Interface motion and equilibrium were studied within the setting of a one-dimensional chain of harmonic oscillators. The interface demarcates regions of differing elastic stiffness and accretes through isothermal fluctuation theory, while the motion of each mass is governed by a simple force balance.

The stress gradients in CrN films are described by the net sum of three contributions, which includes tensile stress, compressive stress and thermal stress. The tensile stress due to grain boundaries depends on the height in the film, the thermal tensile stress is not dependent on film thickness and compressive stress is caused by ion peening.

X-ray diffraction mapping is used to show concave crystal plane curvature in silicon carbide substrates from five commercial suppliers with two suppliers producing wafers with

A report on fine-grained polysilicon layers with a grain size of only 0.1-0.2 [mu]m that shown an open-circuit voltage of 536 mV is presented. Phosphorus spikes along the grain boundaries enhance the junction area and determine the collection and the recombination volumes.

The Nd self-diffusivity is studied in nanocrystalline Nd-rich Nd2Fe14B. Grain-boundary diffusion coefficients D(super Nd)(sub GB) are derived from the analysis of the diffusion profiles.

A simple method to estimate the effective biaxial modulus of cubic polycrystalline films with an arbitrary [hkl] fiber texture is presented. For an arbitrary [hkl] fiber axis, analytic expressions are given for the Voigt and Reuss bounds, followed by proving that these two bounds coincide if, and only if, the fiber axis is equivalent to [100] or [111] when the constituent grains are anisotropic.

The Superconducting Quantum Interference Filters (SQIFs) were proposed as high sensitive magnetic field detectors. SQIFs are arrays of Josephson junctions (JJs) with a specially selected distribution of the loop areas, such that the magnetic field dependent voltage response V(B) is a delta-like dip increases with the number of loops connected in series.

A comparative study of the electromagnetic parameters of YBa2Cu3O(sub 7-x) grain boundary Josephson Junctions (GBJJs) fabricated on 45-degree [100] asymmetric and 24-degree [001] symmetric tilt bicrystalline substrates are presented. It is observed that the capacitance values deduced by means of Fiske steps in YBCO 45 degree [100] asymmetric and 24 degree [001] symmetric GBJJs scale with junction resistance in opposite direction.

The article, on the basis of the Boltzmann equation, proposes a model that includes scattering from both film surfaces and grain boundaries. The quasiclassical electrical transport in metallic films is studied.

Presenting the near-field microwave microscope is capable of identifying local nonlinear sources, such as a bi-crystal grain boundary and edge-effects, with a spatial resolution similar to the loop probe size is discussed. Preliminary results on the nonlinear properties of some commonly used substrates, e.g., MgO and STO, have also been obtained.

Investigating an effective treatment for the analysis of an anisotropic type II polycrystalline material is discussed. On this basis it is proposed that vortex pinning in polycrystalline MgB2 is due to the difference of 'effective' k across grain boundaries.